Studies on Mesogenic Disc-like Molecules III. Heat Capacity of Benzene-hexa-n-octanoate from 13 to 393 K

Abstract

The heat capacity of benzene-hexa-n-octanoate, C₆(OCOC₇H₁₅)₆, with a purity of 99.77 mole per cent has been measured with an adiabatic-type calorimeter in the range from 13 to 393 K. Three phase transitions were found at 301.89 (Phase II → I), 355.10 (Phase I → columnar mesophase) and 357.09 K (columnar mesophase → isotropic liquid). The enthalpy and entropy of these transitions were 48.96 kJ mol₋₁/164.01 J K₋₁ mol⁻¹, 46.07/129.81 and 19.22/53.77, respectively. The infrared spectra recorded in the range 4000−30 cm₋₁ showed a drastic change between the two crystalline phases. The spectra of the mesophase were substantially the same as those of the isotropic liquid. As in the case of C₆(OCOC₆H₁₃)₆, the heat capacity of the mesophase was much smaller than those of the adjacent crystalline and isotropic liquid phases. This compound also acquired a large amount of transition entropy due to conformational melting of the paraffinic moieties in the solid state. The flexible paraffinic moieties play an important role for the appearance of the columnar mesophase.